The present invention relates generally to fuel injection equipment for internal combustion engines and relates more particularly to apparatus for controlling the rate of injection in accumulator type injectors.
In an accumulator type fuel injection system, fuel is accumulated under high pressure in the injection nozzles and means are provided for holding the valve needle closed against the pressurized fluid until the proper time to initiate injection. In a typical accumulator system, the additional nozzle closing force which supplements the valve spring force is in the form of a high fuel pressure acting against the upper end of the valve, the release of which permits the valve to open under the influence of the accumulated fuel pressure.
A shortcoming of accumulator type injectors is their characteristic high initial injection rate which drops continuously through the injection interval. This is an undesirable rate characteristic since in most cases, it is desirable to initiate injection at a substantially lower than maximum rate, in some cases even using a pilot injection, to optimize the combustion process.
The present invention is particularly adapted for use with the type of accumulator injector which employs an amplifier piston to hydraulically raise the pressure of the fuel delivered to the accumulator chamber by the rail pump to a high level which might be ten to twenty times the pressure delivered by the pump. An example of such an injection system is shown in U.S. Pat. No. 4,628,881, issued on Dec. 16, 1986. In this system, a fuel supply rail pressure on the order of 1,000 psi is supplied continuously to unit injectors which through appropriate passage and valve means assure a continuous accumulator chamber pressure substantially equal to the rail pressure and well below the pressure required to open the nozzle valve against the closing spring. A double acting solenoid valve when actuated admits rail pressure to the upper end of a pressure amplifying piston assembly which acts to increase the fuel pressure in the accumulator chamber, for example, by a factor of 15. A check valve member disposed concentrically around the upper end of the injection valve needle permits the boosting of the pressure in the accumulator chamber but prevents any backflow when the amplified pressure is released. The upper end of the valve needle extends through the check valve element and is subjected to the amplified pressure, thus supplementing the valve needle closing spring and holding the valve needle closed until the amplified pressure is released.
Upon deenergizing of the solenoid, the double acting valve closes the rod piston inlet to the amplified piston and opens the piston to drain, resulting in a rapid decompression of the fuel pressure above the check valve, thus allowing the valve needle to move sharply upwardly under the force of the accumulated fuel charge. Upon the fuel pressure in the accumulator chamber dropping to the nozzle closing pressure, the nozzle closes under the spring force. This cycle is repeated for each firing sequence of the engine cylinder.